Doxorubicin and its analogs are important anthracycline antibiotics used in cancer chemotherapy. Tumor cell resistance to doxorubicin is often related to reduced influx and/or enhanced efflux. We have used laser flow cytometry and clonogenic assays to study anthracycline transport, retention and cytotoxicity in murine leukemic cells. We have developed methods for rapid identification and laser activated sorting of drug resistant cells on the basis of their drug retention and presence of resistance felated markers. our preliminary studies on human solid tumors show that unlike murine in vitro cell lines, cells from these tumors express extreme heterogeneity in drug ratention and sensitivity to drugs which block drug efflux and enhance retention and cytotoxicity. In this revised competitive renewal, we propose to extend our multifaceted studies on murine cells to selected human melanoma cell lines and their xenografts. These tumor lines cover a wide range of doxorubicin sensitivity from very low to very high resistance. We propose to use our rapid methods to correlate drug retention, sensitivity to drug efflux blockers (phenothiazines), presence or absence of various biochemical and biophysical markers of resistance with effects on clonogenecity and xenograft growth inhibition. We will carry out specific studies to delineate the role of drug transport, DNA damage/repair, and scavenging of free radicals in tesistance of human melanomas to anthracyclines. It is hoped that use of our rapid analytical methods will allow us to better understand anthracycline effects on proliferation, correlation between drug retention, presence or absence of resistance related markers, and cellular resistance of human melanoma cells to this important class of chemotherapeutic agents.